1. Field of the Invention
The present invention relates to a medical image diagnosis apparatus and the control method thereof that forms a medical image of an object. In particular, it relates to techniques that correct displacement of an image caused by bending of the patient table (patient table deflection) on which an object is placed.
2. Description of the Related Art
Conventionally, medical image diagnosis apparatuses such as the X-ray CT apparatus and nuclear medical diagnosis apparatus (PET, SPECT, etc.) have been widely used (refer to Japanese Unexamined Patent Application Publication No. 2004-180846 as an example). In recent years, diagnostic systems that combine an X-ray CT apparatus with a nuclear medical diagnosis apparatus, such as a PET-CT, are also in practical use (refer to Japanese Unexamined Patent Application Publication No. 2005-291814 as an example).
An example of construction of a general medical image diagnosis apparatus (X-ray CT apparatus) is shown in FIG. 1 and FIG. 2. This X-ray CT apparatus 1000 comprises a gantry 2, examination table 3, computer 4, monitor 5, and input device 6.
The monitor 5 and input device 6 are used as a console 7 for the X-ray CT apparatus 1000 (refer to FIG. 2). The monitor 5 comprises any display device such as an LCD (Liquid Crystal Display) or CRT display (Cathode Ray Tube display). The input device 6 comprises any input device, including a keyboard, mouse, trackball, control panel, touch panel, etc.
The gantry 2 houses a turnable support 21 as shown in FIG. 2. The X-ray tube 22 and X-ray detector 23 are supported by the support 21. The X-ray tube 22 generates X-rays based on a specified tube voltage and the tube current applied by a high-voltage transformer assembly 24 and delivers an X-ray fan-beam and cone-beam to an object P located inside the opening 2A of the gantry 2. The X-ray detector 23 is supported at a location opposite the X-ray tube 22 over the opening 2A. The X-ray detector 23 comprises arrayed multiple X-ray detection elements that detect dosage of the X-ray beam transmitted at the object P.
The support 21 is rotated along the circumference of the opening 2A by a support drive section 25. The X-ray tube 22 and X-ray detector 23 rotate along with the support 21 as it rotates in a unified manner to scan the object P with X-ray beam. This allows the X-ray dosage of the X-ray beam transmitted at the object P to be detected from various directions. The data (detection signal) of the transmitted X-ray dosage detected by the X-ray detector 23 is sent to data acquisition part 26.
The data acquisition part 26 is a so-called DAS (Data Acquisition System) that comprises arrayed data acquisition elements, similar to each X-ray detection element of the X-ray detector 23, which collect data (detection signal) of the transmitted X-ray dosage detected by the X-ray detector 23. The data acquisition part 26 performs amplification and A/D conversion processing of the collected data and transmits the data to the computer 4.
The support drive section 25 not only rotates the support 21 as described above but also operates to tilt the support 21 toward the object P.
The examination table 3, as shown in FIG. 1, comprises a patient table 31 on which an object P is placed and an examination table base 32 that supports the patient table 31. The examination table base 32 houses a patient table drive 33 (refer to FIG. 2) that moves the patient table 31 variously in an anteroposterior direction (in the direction of the arrow in FIG. 1, horizontal direction; z-direction), up and down direction (vertical direction; y-direction), longitudinal direction (horizontal direction perpendicular to the anteroposterior direction; x-direction). The abovementioned anteroposterior direction (z-direction) is the direction of the body axis of the object P on the patient table 31.
The computer 4 comprises, for example, a general-purpose computer. The computer 4 houses a microprocessor such as a CPU, memory such as RAM or ROM, a high-capacity storage unit such as a hard disk drive, and an interface that sends and receives data to and from other devices. The other devices may be a gantry 2, examination table 3, console 7, and another computer on a network not shown here, etc.
The computer 4 comprises a device control part 41 and image processing part 42. The device control part 41 controls the operation of each part of the X-ray CT apparatus 1. For example, the device control part 41 executes control of rotation and tilting of the support 21 by the support drive section 25, operational control of the X-ray tube 22 by a high-voltage transformer assembly 24, operational control of the X-ray detector 23, operational control of the data acquisition part 26, moving operational control of the patient table 31 by the patient table drive 33, etc.
The image processing part 42 applies preprocessing to the transmitted X-ray dosage data collected by the gantry 2 to generate projection data. Furthermore, the image processing part 42 reconstructs the image data of the tomographic image of the object P based on the projection data.
For such medical image diagnosis apparatus, the problem of deflection off the patient table 31 caused by the weight of the object P has been indicated as described in Japanese Unexamined Patent Application Publication No. 2004-180846 and Japanese Unexamined Patent Application Publication No. 2005-291814, etc. More specifically, when the object P is not placed on the patient table 31, the patient table 31 does not bend (significantly) when moving the patient table 31 in the z-direction (refer to FIG. 3A). However, when the object P is placed thereon, the end side of the patient table 31 (the side near the opening of the gantry 2) is bent downward (y-direction) because of the weight of the object P (refer to FIG. 3B). The quantity of deflection (downward displacement, etc.) of the patient table 31 varies, depending on the body weight, etc., of the object P and the position where the object P is placed on the patient table 31. The quantity of deflection also varies, depending on the position of the patient table 31 (distance of the patient table 31).
When X-ray beam scanning is performed with the patient table 31 bent in this manner, the body axis of the object P, which should be horizontal, is placed tilted at the opening 2A. Then, as shown in FIG. 4, displacement Δy in the y-direction occurs at the slice location A, and the displacement Δy will be reflected in the reconstruction image (where Δy=y−y0:y0=y-coordinate value of the patient table 31 when the object P is not placed thereon; y=y-coordinate value of the patient table 31 when the object P is placed thereon).
Moreover, the cross-section at the slice location A, which is preset in the planning stage of scanning, will be tilted as shown in FIG. 3B, because the body axis of the object P tilts along with the obliquity of the patient table 31. With imaging in such a tilted position, the tomographic image of the cross-section at the slice location α, shown in FIG. 3B, will be reconstructed.
As described above, making an image diagnosis using a less-accurate reconstructing image different from the cross-section at the planning stage of the scan may lead to inaccuracy in diagnosis. For example, there is concern about deterioration of treatment planning for radiotherapy whereby radiation is delivered to an affected area such as a tumor.
Particularly, in recent years, a small lesion can be detected with the improvement of the image resolution of the medical image diagnosis apparatus. In order to deliver radiation to this tiny target accurately, the location of the target should be precisely pinpointed from an image, and then the actual target location in the object P corresponding to the pinpointed location needs to be specified.
When using an image containing a displacement caused by patient table deflection, it is relatively easy to pinpoint the location of a lesion in the image, but it is difficult to specify the location in the object P with a high degree of accuracy in relation to the location pinpointed from the image, because a state in which the object is placed on the patient table of the object P is different from a state in which the object is placed on the patient table of the object P during the treatment planning stage (in general, the object P is placed with his/her body axis being horizontal).
A wide variety of efforts have been taken to deal with such patient table deflection, for example, placing a member (a patient table support member such as a shore) that supports the patient table 31, or that the treatment planner properly adjusts the X-ray irradiated site by considering the effect of the patient table deflection.
Meanwhile, radiation therapy called IMRT (Intensity Modulated Radiation Therapy) has been performed in recent years. IMRT combines multiple beams to allow radiation to adjust its level, so tumor tissue is exposed to radiation intensively whereas the adjacent normal tissue receives lower irradiance level. This allows stronger radiation to be delivered to the tumor without increasing side effects.
In IMRT, an image is taken with a medical image diagnosis apparatus first to specify the location and shape of the tumor, and the irradiated area and intensity will be determined accordingly. Next, mark the body surface of the object and take a medical image to confirm the irradiated area. The irradiated area and intensity will be adjusted if needed (it is called positioning). Then, the object is placed on a special treatment device to perform radiation therapy.
Marking is done by a seal applied to the body surface or drawing a mark with a pen. There are visible markings that appear in a medical image as well as invisible markings that do not. For the former, for example, a seal made of a material with X-ray absorption that is different from that of human body and patient table.
When using an X-ray CT apparatus, for example, mark three points on the body surface to specify a location to be matched with the scanning center (the rotation center of the X-ray tube and X-ray detector) and capture an image, figure out the location and shape of the tumor, and confirm the irradiated area, etc. The location of the tumor and the irradiated area will be learned as a specified location by the marking, that is, the displacement from the scanning center. Marking may be applied to indicate the irradiation center (isocenter).
A construction is disclosed to Japanese Unexamined Patent Application Publication No. 2004-180846 that detects the displacement of the patient table and corrects the relative position of the patient table and the slice direction based on the detected result. The displacement of the patient table here is detected by using sensors installed in a longitudinal direction at specified distances on the patient table and a CCD camera that captures the condition of the displacement of the patient table. In addition, the relative position is corrected by lifting and lowering the examination table (patient table) and changing the height and tilt angle of the gantry.
A construction is also disclosed in Japanese Unexamined Patent Application Publication No. 2004-180846 that detects the displacement of the patient table by using similar sensors, and based on the detected result, extracts an image data of which the relative position of the patient table and the scan location is corrected from the multiple image data collected at multiple locations.
An invention described in Japanese Unexamined Patent Application Publication No. 2005-291814 is related to aligning image data collected from each medical image diagnosis apparatus for complex diagnosis system such as PET-CT.
Specifically, first, the location information of the projection data for X-ray CT and the location information of absorption compensation data are extracted, and the projection data for nuclear medicine having functional data will be corrected based on this absorption compensation data. Next, based on the displacement of the extracted projection data for X-ray CT and absorption compensation data, both or one of the projection data for X-ray CT and corrected projection data for nuclear medicine (alternatively, move both or one of the tomographic image for X-ray CT and corrected tomographic image for nuclear medicine) are moved. Then, projection data and absorption compensation data are determined by X-ray CT apparatus and nuclear medical diagnosis apparatus respectively with reference to an object having no change in location over time, and projection data and absorption compensation data for nuclear medicine are determined with each radiation that transmitted the same location of the object so that the heightwise displacement between a tomographic image for X-ray CT and a tomographic image for PET caused by the deflection from the patient table is corrected.
A construction is disclosed in Japanese Unexamined Patent Application Publication No. 2005-291814 that installs the same radiation source on a nuclear medical diagnosis apparatus as the radioactive agent that is administered to an object when imaging for nuclear medicine to be irradiated to the object to determine absorption compensation data based on the transmitted radiation delivered to an object.
When dealing with the above mentioned problem of the displacement of the patient table with the conventional medical image diagnosis apparatus described above, the following inconvenience will occur.
First of all, when applying the above mentioned patient table supporting member or the construction with the sensors installed on the patient table (Japanese Unexamined Patent Application Publication No. 2004-180846), a major alteration will be needed for the general hardware configuration of the medical image diagnosis apparatus (e.g., the patient table supporting member and the sensors need to be added.). On the other hand, considering the price of the apparatus, etc., it would be a heavy burden for the user to purchase a new apparatus with such construction preapplied.
A construction is disclosed in Japanese Unexamined Patent Application Publication No. 2004-180846, that deals with the displacement of the patient table by tilting the tablet top and the gantry. When performing a helical scan by an X-ray CT apparatus, for example, as the tilt caused by the displacement of the patient table varies from the location to location of the patient table, the angle of tilting the patient table and gantry should be controlled to change sequentially along with the tilt of the patient table changes. However, it is not easy to control such movements accurately. In addition, when applying the construction of changing the angle of tilting the patient table, the patient as the object may feel uncomfortable as the tilt angle of the object is also changed accordingly.
Japanese Unexamined Patent Application Publication No. 2004-180846 assumes that when calculating the quantity of deflection between the reference positions (according to the document, the positions where the sensors are placed) placed on the patient table, the direction of the displacement of all of each point is parallel.
However, in order to calculate the quantity of deflection accurately based on this assumption, further assumptions are needed including that the quantity of deflection of the each point is negligibly small and the distance between the reference positions is short enough.
When applying the former assumption, the patient table must be strengthened, but if the strength of the patient table is enhanced, other problems may arise, including reduction of sensitivity to gamma ray detection due to the increased absorption of gamma rays by the patient table as described in Japanese Unexamined Patent Application Publication No. 2005-291814.
On the other hand, when applying the latter assumption, multiple sensors should be installed on the patient table which also causes other problems such as increase in cost, complex control and more maintenance.
Furthermore, assuming that the direction of the displacement of all of the each point is parallel, a problem occurs in that the tilt condition of the patient table cannot be accurately detected. Particularly, if the distance between the sensors is large or the object is heavy, the angle of tilting the patient table between the sensors is expected to change. However, the change in the angle of tilting the patient table between the sensors cannot be detected, because only the tilt angle of the line connecting the sensors can be calculated according to the computation method of the document.
In addition, when a treatment planner is dealing with the deflection of the patient table by displacing the x-ray irradiation site by considering the effect of the patient table deflection, the treatment planner has to change the X-ray irradiated site based on his/her own experience, etc., not on objective data of the quantity of deflection. Therefore, it is difficult to ensure the accuracy of the irradiated site.
Moreover, because the construction described in Japanese Unexamined Patent Application Publication No. 2005-291814 is for matching an image for nuclear medicine with an image of an X-ray CT, it cannot be applied to cases in which a standalone X-ray CT apparatus is used.
In addition, when performing IMRT, the irradiated area must be determined precisely and the radiation must be delivered to the tumor accurately. For this purpose, the location in an image must be corresponded to the actual location in the object with a high degree of accuracy. For example, when X-ray CT apparatus is used, the correspondence is ensured by deeming the location specified by marking to be the scanning center.
In IMRT, however, because images are taken with a medical image diagnosis apparatus and radiation irradiation is performed with a special treatment device, a state in which the object is placed on the patient table when radiation is delivered to the object may be different from the condition when the image is taken and the irradiated area specified by the image may not match the actual irradiated area because of the effect by the patient table deflection. Such misalignment not only prevents intensive radiation delivery to the tumor but also exposes normal tissue to radiation. In particular, it is difficult to detect misalignment of the irradiated area when the marking is invisible.